Combination thermal wave and optical spectroscopy measurement systems

ABSTRACT

A combination metrology tool is disclosed which is capable of obtaining both thermal wave and optical spectroscopy measurements on a semiconductor wafer. In a preferred embodiment, the principal combination includes a thermal wave measurement and a spectroscopic ellipsometric measurement. These measurements are used to characterize ion implantation processes in semiconductors over a large dosage range.

This is a continuation of U.S. patent application Ser. No. 09/499,974,filed Feb. 8, 2000 now U.S. Pat. No. 6,535,285.

TECHNICAL FIELD

The subject invention relates to a method and apparatus particularlysuited for the analysis ion implantation at higher doses onsemiconductor samples.

BACKGROUND OF THE INVENTION

There is a great need in the semiconductor industry for metrologyequipment which can provide high resolution, nondestructive evaluationof product wafers as they pass through various fabrication stages. Inrecent years, a number of products have been developed for thenondestructive evaluation of semiconductor samples. One such product hasbeen successfully marketed by the assignee herein under the trademarkTherma-Probe. This device incorporates technology described in thefollowing U.S. Pat. Nos. 4,634,290; 4,636,088, 4,854,710 and 5,074,669.The latter patents are incorporated herein by reference.

The Therma-Probe device monitors ion implant dose using thermal wavetechnology. In this device, an intensity modulated pump laser beam isfocused on the sample surface for periodically exciting the sample. Inthe case of a semiconductor, thermal and plasma waves are generated inthe sample which spread out from the pump beam spot. These waves reflectand scatter off various features and interact with various regionswithin the sample in a way which alters the flow of heat and/or plasmafrom the pump beam spot.

The presence of the thermal and plasma waves has a direct effect on thereflectivity at the surface of the sample. Features and regions belowthe sample surface which alter the passage of the thermal and plasmawaves will therefore alter the optical reflective patterns at thesurface of the sample. By monitoring the changes in reflectivity of thesample at the surface, information about characteristics below thesurface can be investigated.

In the basic device, a second laser is provided for generating a probebeam of radiation. This probe beam is focused colinearly with the pumpbeam and reflects off the sample. A photodetector is provided formonitoring the power of reflected probe beam. The photodetectorgenerates an output signal which is proportional to the reflected powerof the probe beam and is therefore indicative of the varying opticalreflectivity of the sample surface.

The output signal from the photodetector is filtered to isolate thechanges which are synchronous with the pump beam modulation frequency.In the preferred embodiment, a lock-in detector is used to monitor themagnitude and phase of the periodic reflectivity signal. This outputsignal is conventionally referred to as the modulated opticalreflectivity (MOR) of the sample.

Thermal wave technology is well suited for measuring lattice damage incrystalline materials and, therefore, serves as an excellent technologyfor monitoring the ion implant process in semiconductor materials. It isalso known that optical methods, such as spectroscopic reflectance andspectroscopic ellipsometry, are sensitive to lattice damage through theeffects of such damage on the optical properties of the material beingimplanted.

Typically, thermal waves are more sensitive in the region of lowimplantation, i.e. less than 10¹² ions/cm² (arsenic at 30 KEV) than theoptical methods. In the range of 10¹² through 10¹⁴ ions/cm², it appearsthat optical and thermal waves are comparable in their ability to detectchanges in lattice damage. At higher doses (of the same implant),amorphization sets in and the thermal wave signal is no longer monotonicwith increasing dose and cannot be used reliably to monitor the implantprocess. In this high dose region, the optical methods are verysensitive and can unambiguously measure the thickness of the amorphouslayer.

There is, however, damage above and below the amorphous layer which canstill make an accurate measurement of total damage in the implantedmaterial difficult using only optical methods. More specifically, theimplantation process at high doses will create a large damaged regionwith a relatively smaller layer of amorphous material in the centerthereof. This occurs because during the implantation process, the ionstravel very quickly as they first strike the lattice. The fast passagethrough the lattice can result in little or no damage immediatelybeneath the surface. As the ions begin to slow down, the damageincreases until at a certain depth, the damage is sufficient to produceamorphization. Amorphization represents the peak damage to the lattice.Ions which travel beyond the amorphous layer will cause further damage,but below the threshold for amorphization. Semiconductor manufacturersare interested in knowing both the thickness of the amorphous region, aswell as the total extent of damage to the lattice which would includethe damaged regions both above and below the amorphous layer.

Thermal waves are intrinsically more sensitive to total damage than theoptical methods. Therefore, by combining thermal waves withspectroscopic measurements, one can provide a means for sensitive andunambiguous monitoring of the ion implant process throughout the entirerange. More specifically, one can use the data derived from the thermalwave measurements to provide an indication of the full extent of thedamaged region. Data obtained from a spectroscopic measurements can beused to provide an indication of the thickness of the amorphous layer.By combining these two sets of measurements, one can provide an accurateprofile of the damage as a function of depth below the surface of thesemiconductor wafer.

The concept of combining thermal wave measurements with other opticalmeasurements is disclosed in prior U.S. Pat. No. 5,978,074, issued Nov.2, 1999, and is assigned to the same assignee as the subject inventionand is incorporated herein by reference. This patent describes the needto obtain additional measurements where the sample is more complex. Inone aspect of that patent disclosure, a conventional thermal wavedetection system was modified to increase the amount of data which couldbe obtained. For example, a steering system was provided for varying thedistance between the pump and probe beam spots as measurements weretaken. Another approach was to obtain a sequence of measurements atvarious pump beam modulation frequencies.

The prior patent also discussed the advantages of combiningspectroscopic reflectivity measurements with the thermal wavemeasurements. Various additional measurements were suggested includingthe assignee's proprietary beam profile reflectometry and beam profileellipsometry techniques. The latter two approaches are described in U.S.Pat. Nos. 4,999,014 and 5,181,080, both of which are incorporated hereinby reference.

The principal application for the tool described in U.S. Pat. No.5,87,974 relates to measuring thin metal films formed on semiconductorsamples. The latter patent did not disclose the advantages of combiningthermal wave measurements with spectroscopic ellipsometry measurements.Further, the latter patent did not discuss the specific concept of usinga thermal wave measurement to provide information on the full extent ofa damage layer, while using another optical measurement to provide anindication of the amorphous layer.

Accordingly, it is an object of the subject invention to provide a newmethod and apparatus which provides additional measurement capabilities.

It is another object of the subject invention to provide a method andapparatus particularly suited to evaluating high dopant concentrationsin semiconductors.

It is a further object of the subject invention to provide a method andapparatus which combines measurements of modulated optical reflectivitywith modulated spectroscopic ellipsometry.

SUMMARY OF THE INVENTION

In accordance with these and other objects, the subject inventionincludes a method wherein a sample is characterized through acombination of measurements which include both a thermal or plasma wavemeasurement and a spectroscopic measurement. The thermal/plasma wavemeasurement is obtained by periodically exciting a region on the samplewith an intensity modulated pump beam. A probe beam is directed to aregion on the sample surface which has been periodically excited.Changes in power of the reflected probe beam are monitored to obtain themodulated optical reflectometry of the sample.

In accordance with the subject method, a separate spectroscopicmeasurement is also obtained. To obtain this measurement, apolychromatic light source generates a polychromatic probe beam which isdirected to reflect off the sample. The intensity of the reflectedpolychromatic probe beam can be measured to obtain spectroscopicreflectance data. Alternatively, or in addition, the change inpolarization state of the polychromatic probe beam can be measured toobtain ellipsometric information. Additional measurement technologiescan also be employed.

In accordance with the subject invention, data corresponding to themodulated optical reflectivity signal is combined with the spectroscopicdata to more accurately characterize the sample. In one preferredembodiment, the system is used to more fully characterize high dosagelevels of ion implantation in a semiconductor wafer. In this approach,the modulated optical reflectivity signal is useful for characterizingthe full extent of the damaged region, while the spectroscopicellipsometric information is used to characterize the extent of theamorphous layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the apparatus for carrying out themethods of the subject invention.

FIG. 2 is a graph illustrating the damage layer thickness across a fullion implant dose range as measured by a spectroscopic ellipsometer.

FIG. 3 is a graph illustrating the thermal wave response across a fullion implant dose range.

FIG. 4 is a statistical process control plot of long term monitoring ofa silicon wafers implanted with arsenic using the three differentmeasurement technologies.

FIG. 5 is a statistical process control plot of long term monitoring ofa silicon wafers implanted with arsenic using the three differentmeasurement technologies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a simplified diagram of the basic components of an apparatus10 which can be used to take the measurements useful in applying themethods of the subject invention. The apparatus is particularly suitedfor measuring characteristics of semiconductor wafers 20. In oneimportant aspect of the invention, the device is used to characterizelevels of ion implantation in the wafer. The device could also be usedto characterize properties of one or more thin film layers 22 on top ofthe wafer.

In accordance with the subject invention, the apparatus includes a firstmeasurement system for generating thermal and/or plasma wavers andmonitoring the propagation of these waves in the sample. This portion ofthe system includes a pump laser 30 for exciting the sample and a probelaser 32 for monitoring the sample. Gas, solid state or semiconductorlasers can be used. As described in the assignee's earlier patents,other means for exciting the sample can include different sources ofelectromagnetic radiation or particle beams such as from an electrongun.

In the preferred embodiment, semiconductor lasers are selected for boththe pump and probe lasers due to their reliability and long life. In theillustrated embodiment, pump laser 30 generates a near infrared outputbeam 34 at 790 nm while probe laser 32 generates a visible output beam36 at 670 nm. Suitable semiconductor lasers for this application includethe Mitsubishi ML6414R (operated at 30 mW output) for the pump laser anda Toshiba Model 9211 (5 mW output) for the probe laser. The outputs ofthe two lasers are linearly polarized. The beams are combined with adichroic mirror 38. It is also possible to use two lasers with similarwavelengths and rely on polarization discrimination for beam combiningand splitting.

Pump laser 30 is connected to a power supply 40 which is under thecontrol of a processor 42. The output beam of laser 30 is intensitymodulated through the output of power supply 40. The modulationfrequency has a range running from 100 KHz to 100 MHz. In the preferredembodiment, the modulation frequency can be set up to 125 MHz. Asdescribed in the above cited patents, if an ion laser is used togenerate the pump beam, the intensity modulation can be achieved by aseparate acousto-optic modulator.

Prior to reaching the beam combining mirror 36, the probe beam 34 passesthrough a tracker 46. Tracker 46 is used to control the lateral positionof beam 34 with respect to the probe beam. In some measurements, the twobeams will be positioned so that the spots will overlap on the samplesurface. In addition, measurements can be taken at various spacingsbetween the pump and probe beam spots. Measurements at different spatialseparations are discussed in greater detail in U.S. Pat. No. 5,978,074.

The beams are directed down to the sample 20 through a microscopeobjective 50. Objective 50 has a high n.a., on the order of 0.9, and iscapable of focusing the beam to a spot size on the order of a fewmicrons and preferably close to one micron in diameter. The spacingbetween the objective and the sample is controlled by an autofocussystem not shown herein but described in U.S. Pat. No. 5,978,074.

The returning reflected beams 34 and 36 are reflected by beam splitter52. A filter 54 is provided to remove the pump beam light 34 allowingthe probe beam light to fall on the photodetector 60. Detector 60provides an output signal which is proportional to the power of thereflected probe beam 36. Detector 60 is arranged to be underfilled sothat its output can be insensitive to any changes in beam diameter orposition. In the preferred embodiment, detector 60 is a quad cellgenerating four separate outputs. When used to measure reflected beampower, the output of all four quadrants are summed. As described in U.S.Pat. No. 5,978,074, the apparatus can also be operated to measure beamdeflection. In the latter case, the output of one adjacent pair ofquadrants is summed and subtracted from the sum of the remaining pair ofquadrants.

The output of the photodetector 60 is passed through a low pass filter72 before reaching processor 42. One function of filter 72 is to pass asignal to the processor 42 proportional to the DC power of the reflectedprobe. A portion of filter 72 also functions to isolate the changes inpower of the reflected probe beam which are synchronous with the pumpbeam modulation frequency. In the preferred embodiment, the filter 72includes a lock-in detector for monitoring the magnitude and phase ofthe periodic reflectivity signal. Because the modulation frequency ofpump laser can be so high, it is preferable to provide an initialdown-mixing stage for reducing the frequency of detection. Furtherdetails of the preferred filter and alternatives are described in U.S.Pat. No. 5,978,074. For example, it would be possible to use a modulatedpump beam to obtain an optically heterodyned signal as described in U.S.Pat. No. 5,206,710, incorporated herein by reference.

To insure proper repeatability of the measurements, the signals must benormalized in the processor. As noted above, the DC reflectivity of theprobe beam is derived from detector 60. In addition, the DC outputpowers of the pump and probe lasers are monitored by incident powerdetectors (not shown) and provided to the processor. The signals arefurther normalized by taking a measurement of the power of the pump beam34 after it has been reflected by another detector (not shown). Thismeasurement is used to determine the amount of pump energy which hasbeen absorbed in the sample. The DC signal for both the incident pumpand probe beam powers as well as the reflected beam powers are used tocorrect for laser intensity fluctuations and absorption and reflectionvariations in the samples. In addition, the signals can be used to helpcalculate sample parameters.

In accordance with the subject invention, in addition to the thermalwave measurement system, a separate spectroscopic measurement system isalso included. This additional system includes a polychromatic or whitelight source 80. The white light source can be defined by a singlebroadband lamp, such as a xenon arc lamp. Alternatively, the white lightsource could be defined by two or more lamps such as a xenon arc lamp tocover of the visible light ranges and a separate deuterium lamp to coverthe ultraviolet ranges.

The output from the white light source 80 is a polychromatic probe thebeam 82. The beam can be redirected by a splitter 84 towards the sample.The beam 82 is focused onto the sample by microscope objective 50. Thereflected beam is redirected by splitter 86 to a spectrometer 88. Thespectrometer can be of any type commonly known and used in the priorart. In the illustrated embodiment, the spectrometer includes a curvedgrating 90 which functions to angularly spread the beam as a function ofwavelengths. A photodetector 92 is provided for measuring the beam.Photodetector 92 is typically a photodiode array with differentwavelengths or colors falling on each element in the array. Otheralternative detectors would include a CCD camera or photomultiplier. Itshould be noted that it is within the scope of this invention to use amonochrometer and obtain measurements serially (one wavelength at atime) using a single detector element.

The output of detector 92 is supplied to the processor 42. When thepolychromatic light beam 82 follows the path discussed above, the outputof detector 92 would correspond to the reflectance of the sample. Inaccordance with the subject invention, polychromatic light beam 82 canalso be used to obtain spectroscopic ellipsometric measurements.

In order to obtain spectroscopic ellipsometric measurements, a beamsplitter 102 can be placed in the path of the polychromatic light beam82. Beams splitter 102 redirects the beam through polarizer 106 tocreate a known polarization state. Polarizer 106 can be a linearpolarizer made from a quartz Rochon prism. The polarized probe beam isfocused onto the sample 20 by a curved mirror 108. The beam strikes thesample at an angle on the order of 70 degrees to the normal to maximizesensitivity. Based upon well-known ellipsometric principles, thereflected beam will generally have a mixed linear and circularpolarization state after interacting with the sample, as compared to thelinear polarization state of the incoming beam. The reflected beam isredirected by mirror 110 through a rotating compensator 112. Compensator112 introduces a relative phase delay or phase retardation between apair of mutually orthogonal polarized optical beam components. Theamount of phase retardation is a function of the wavelength, thedispersion characteristics of the material used to form the compensatorand the thickness of the compensator. The compensator is rotated bymotor 116 at an angular velocity ω about an axis substantially parallelto the propagation direction of the beam. In the preferred embodiments,compensator 112 is a bi-plate compensator constructed of two parallelplates of anisotropic (usually birefringent) material, such as quartzcrystals of opposite handedness, where the fast axes of the two platesare perpendicular to each other and the thicknesses are nearly equal,differing only by enough to realize a net first-order retardation overthe wavelength range of interest.

After passing through the compensator 112, the beam interacts with theanalyzer 120. Analyzer 120 service to mix a polarization states of thebeam. In this embodiment, analyzer 120 is another linear polarizer. Therotating compensator spectroscopic ellipsometer illustrated herein isdescribed in greater detail in U.S. Pat. No. 5,973,787 assigned to thesame assignee and incorporated herein by reference. While a rotatingcompensator ellipsometer is disclosed, the scope of the subjectinvention is intended to include any of the other conventionalspectroscopic ellipsometer configurations. These would include rotatinganalyzer systems as well as fixed element systems that rely onphotoelastic modulators for retardation.

After the beam passes analyzer 120 it is reflected by beam splitter 130and directed to the spectrometer 88. As noted above, grating 90disperses the beam onto the array detector 92. The measured output fromthe spectrometer corresponds to the change in polarization state of thebeam and from this information, the traditional ellipsometric parametersΨ and Δ can be derived.

The optical layout in FIG. 1 is intended to illustrate how both athermal wave detection system and a spectroscopic detection system, andin particular, a spectroscopic ellipsometric system might be employed toobtain measurements at generally the same spot on the surface of thesample and in a near contemporaneous fashion. In this manner, thecombination of the measurements results will produce a more accurateresult.

The combination of the two metrology devices in a single tool inaddition to providing more accurate results provides economic benefitsas well. For example, a single tool has a smaller footprint andtherefore takes up less space in the semiconductor fab. By combiningtechnologies in a single tool, costs can be reduced by eliminatingduplicate subsystems such as wafer handlers and computers. Finally, thecombination can simplify and streamline decision making since theinformation from the two measurement modalities can be coordinatedinstead of producing conflicting results which can occur if two separatedevices were used.

It is believed that a combination thermal wave and spectroscopicinspection device will be particularly useful for analyzing ionimplantation in semiconductor samples. To determine the efficacy of thisapproach, experiments were run to obtain data on similarly preparedwafers on the assignee's Therma-Probe thermal wave device (ModelTP-500), as well as two of assignee's Opti-Probe models, the 3260 forthe broadband spectroscopic measurements and the 5240 to obtainbroadband spectroscopic ellipsometric measurements.

In these experiments, a total of 23 wafers each having a diameter of 150mm were prepared on a GSD-600 ion implanter using P⁺ ions at 100 keVenergy or As⁺ ions at 30 keV. Each wafer had a uniform dose with 20wafers having the P⁺ dose within the range from 1e10 to 1e16 ions/cm²and three wafers having the As⁺ ion dose of 5.4e12±5% ions/cm². Thewafers were not treated for thermal annealing of the crystalline damage.21-point map broadband (210-780 nm) spectroscopic ellipsometry (SE)measurements were performed on the Opti-Probe 5240 and thermal wavemeasurements were done on the Therma-Probe 500 (TP-500) ion implantmonitoring tool. For added comparison, broad band (190-780 nm) spectralreflectance (SR) was done on an Opti-Probe 3260 (OP-3260).

As a part of this investigation, measurement recipes were developed forthe OP-5240 and OP-3260 while the ion implant monitoring recipes werereadily available on the TP-500. For ion implant monitoring purposes, asimple model was applied with a single damage layer on undopedcrystalline silicon and with an overlayer of thermal oxide. The wafersunder study had a nominal 100 Å of thermal oxide deposited prior to ionimplantation. The optical properties of thermal oxide are well known andthe standard library lookup table values were used for the oxidedispersion while the thickness of the top oxide layer was allowed tovary as a fitting parameter. A critical point model with five harmonicoscillators was used for the optical dispersion of the damage layer.Initial recipe development involved fitting of the experimental datawith variable dispersion for the damage layer as well as the damagelayer and the oxide thicknesses.

The phase transition from crystalline-like to amorphous-like silicon wasobserved between doses 2e14 and 4e14 ions/cm² for the Phosphorousimplant at 100 keV. The dose range can then be divided into a low doseregion (1e 10-1e14), a medium dose region (1e14-6e14) and a high doseregion (6e14-1e16). A separate recipe is needed for the curve fitting ofeach dose region due to the limited dynamic range of the fittingalgorithm. FIG. 2 shows the results of curve fitting for the full rangeof ion doses characterized with spectroscopic ellipsometry. FIG. 3 hasthe results for the thermal wave technique. The characteristic lowsensitivity plateau region around the 700 TW unit is missing in the SEresults. Furthermore, the negative slope at higher doses, which isrelated to the optical interference effects in the amorphous layer, isnot an issue for the SE measurements.

In addition to the dose sensitivity, the instrumental noise needs to beincluded in any analysis of implant monitoring capability. The dosesensitivity S in units of %-change in a monitoring parameter per%-change in ion dose can be estimated from$S = \frac{\left( {P_{2} - P_{1}} \right)/\left( {P_{2} + P_{1}} \right)}{\left( {D_{2} - D_{1}} \right)/\left( {D_{2} + D_{1}} \right)}$

where P refers to the monitoring parameter, D is the ion dose, and thesubscript 1 and 2 refer to the samples in question.

To estimate the noise for each technology, the short term repeatabilityof map measurements was determined within a one to two hour period with10 cycles of wafer loading, and the long term repeatability wasestimated from map measurements repeated once a day for five consecutivedays. The repeatability is defined as the standard deviation at 1-σ andthe percent notation %σ=1−σ/mean mean is used here. For each technology,the detection limit DL in %-change in dose can then be estimated from${DL} = \frac{\% \sigma}{S}$

The monitoring results for four sample wafers are illustrated in FIG. 4with the statistical process control (SPC) plots. The data in Tables 1and 2 are analyzed to extract the low dose and high dose detectionlimits in %-dose for each technology. The spectroscopic ellipsometrytechnology shows superior performance in the high dose detection with afactor of 30 improvement in comparison to the industry standard thermalwave technology. For low dose detection the SE performance is comparableto TP-500.

Table 1 below illustrates the long term detection at high dose with eachtechnology. The monitoring parameter for TP-500 is the thermal wavesignal in TW units and for the OP-tools the monitoring parameter is thedamage layer thickness in Å.

HIGH DOSE TP-500 OP-5240* OP-3260 P 100 keV Average 7825.93 1120.361173.265 2.00E+15 1-σ 205.066 0.271 0.802 % 1-σ 2.620 0.024 0.068 P 100keV Average 24667.05 2793.66 2795.649 4.00E+15 1-σ 62.734 0.568 0.589 %1-σ 0.254 0.020 0.021 Sensitivity (%-per-%) 1.555 1.283 1.226 Detectionlimit (% 1.685 0.019 0.056 dose) *3 days of data was acquired

Table 2 below shows the long term detection at low dose with eachtechnology. The monitoring parameter for TP-500 is the thermal wavesignal in TW units and for the OP-tools the monitoring parameter is thedamage layer thickness in Å.

LOW DOSE TP-500 OP-5240* OP-3260 As 30 keV Average 700.61 44.45 42.8255.13E+12 1-σ 0.265 0.130 0.261 % 1-σ 0.038 0.293 0.610 As 30 keV Average704.42 47.71 44.887 5.40E+12 1-σ 0.382 0.235 0.175 % 1-σ 0.054 0.4940.390 Sensitivity (%-per-%) 0.106 1.379 0.917 Detection limit (% 0.5120.358 0.666 dose) *3 days of data was acquired

As can be seen from the above data, both the thermal wave measurementsand the spectroscopic measurements can provide useful information aboution implantation in semiconductor wafers. In accordance with the subjectinvention, this data can be combined in order to improve the analysis ofthe sample and reduced ambiguities.

There are a number of approaches which are available to combine datafrom different technologies. One could use the data independently toarrive at separate approximations of the ion implantation dose of thesample and then derive a final result by taking a weighted average ofthe independent solutions. Preferably, a more robust analysis will beperformed that combines data from both measurements in iterative processto reach a best fit solution. Such iterative approaches for combiningdata from multiple measurements are now more well known in the metrologyfield. For example see “Simultaneous Measurement of Six Layers in aSilicon on Insulator Film Stack Using Spectrophotometry and Beam ProfileReflectometry,” Leng, et. al, Journal of Applied Physics, Vol 81, No. 8,Apr. 15, 1997.

The subject invention is not limited to the particular algorithm used toderive the characteristics of the individual layers. In addition to themore conventional least square fitting routines, alternative approachescan be used. For example, the high level of computing power nowavailable permits approaches to be utilized which include geneticalgorithms. One example of the use of genetic algorithms to determinethe thickness of thin film layers can be found in “Using GeneticAlgorithms with Local Search for Thin Film Metrology,” Land, et. al.,Proceeding of the Seventh International Conference on GeneticAlgorithms, July 19-23, page 537, 1997. See also, U.S. Pat. No.5,864,633, incorporated herein by reference.

In one particular implementation of the subject invention, the thermalwave signals and the spectroscopic ellipsometer signals are combined toanalyze the extent of damage induced by ion implantation. As notedabove, the spectroscopic ellipsometer signals are particularly useful incharacterizing the extent of the amorphous layer created by the ionimplantation process. Conversely, the thermal wave data is particularlysensitive to the overall damage layer. By properly combining the datawith suitable algorithms, a full characterization of the ion implantdamage as a function of depth can be achieved. Alternatively, thespectroscopic reflectivity measurements could be combined with thethermal wave measurements to perform this analysis.

In addition to analyzing ion implantation processes, the combination ofthermal wave and spectroscopic ellipsometric measurements can be used toreduce ambiguities in measurements associated with other aspects ofsemiconductor fabrication. For example, this combination could be usedto analyze characteristics of various thin films of the sample. Suchthin films can include oxides, nitrides and metals. Information obtainedfrom the two or more independent measurements allows one to moreaccurately determined the characteristics of those layers. Thosecharacteristics could include, for example, thickness, index ofrefraction, extinction coefficient, density and thermal conductivity.

Is also within the scope of the subject invention to combine additionaltechnologies to the measurement beyond those illustrated in FIG. 1. Forexample, measurement technologies such as the assignee's proprietarybeam profile reflectometry and beam profile ellipsometry could be used.The addition of such further measurement modules is described inassignee's prior U.S. Pat. No. 5,978,074.

While the subject invention has been described with reference to apreferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

We claim:
 1. A method of evaluating characteristics of a semiconductorwafer wherein the upper region thereof has been implanted with dopants,said method comprising the steps of: periodically exciting a region onthe surface of the wafer; monitoring the modulated optical reflectivityinduced by said periodic excitation and generating first output signalsin response thereto; obtaining spectroscopic ellipsometric informationfrom the same region on the sample surface and generating second outputsignals in response thereto; and evaluating the implantationcharacteristics based on a combination of the first and second outputsignals.
 2. A method as recited in claim 1, wherein said step ofobtaining spectroscopic ellipsometric information comprises the furthersteps of: directing a polychromatic probe beam having a knownpolarization to reflect off the surface of the sample; and monitoringthe change in polarization state of the probe beam induced by reflectionat a plurality of wavelengths.
 3. A method as recited in claim 1,wherein the first and second output signals are combined in an iterativeprocess to find a best fit solution.
 4. An apparatus for evaluating thecharacteristics of a semiconductor wafer wherein the upper regionthereof has been implanted with dopants, said apparatus comprising: anintensity modulated pump laser beam, said pump laser beam being directedto a spot on the surface of the sample for periodically exciting thewafer; a probe laser beam being directed to a spot on the surface of thewafer within a region which has been periodically excited and isreflected therefrom; a detector for measuring the power of the reflectedprobe laser beam and generating a first output signal in responsethereto; a broadband polychromatic light source for generating apolychromatic probe beam having a known polarization state, saidpolychromatic probe beam being directed to reflect off a spot on thesurface of the sample; an analyzer for monitoring the change inpolarization state of the reflected polychromatic probe beam andgenerating a plurality of second output signals corresponding to aplurality of different wavelengths within the polychromatic probe beam;and a processor for filtering the first output signal to provide ameasure of the magnitude or phase of the modulated optical reflectivityof the sample, said processor further functioning to monitor the secondoutput signals and with the first and second output signals beingcombined to evaluate the implantation characteristics of the wafer. 5.An apparatus as recited in claim 4, wherein the probe beam from thepolychromatic light source is directed to the same location as the probelaser beam is directed.
 6. An apparatus as recited in claim 4, furtherincluding a steering means for adjusting the lateral separation betweenthe pump and probe laser beam spots on the surface of the sample andwherein a plurality of measurements are taken at different separationsbetween the pump and probe laser beam spots.
 7. An apparatus as recitedin claim 4, further including a means for varying the modulationfrequency of the pump laser beam and wherein a plurality of measurementsare taken at different modulation frequencies.
 8. An apparatus asrecited in claim 4, wherein the first and second output signals arecombined in an iterative process to find a best fit solution.